1. Field of the Invention
This invention relates to a wafer dicing/die bonding sheet for use in the dicing of silicon wafers and the bonding of chips to lead frames.
2. Background Art
In the prior art, semiconductor devices are generally manufactured by securing a large diameter silicon wafer to a dicing tape, dicing the wafer into semiconductor chips, peeling and removing the chips from the dicing tape, and bonding an individual chip to a lead frame with a curable liquid adhesive.
For simplifying the process and preventing contamination of semiconductor chips with fluid ingredient from the liquid adhesive, there is recently a need for a dicing/die bonding sheet in the form of a pressure-sensitive adhesive sheet that serves as both the pressure-sensitive adhesive layer of dicing tape and the die bonding agent. The dicing/die bonding sheet needs to develop an adhesive (or retentive) force to withstand the dicing operation and to adhere to the chip being taken away in the initial dicing step and needs to further develop a strong bond to the lead frame in the subsequent die bonding step.
JP-A 9-67558 discloses an adhesive for dicing/die bonding sheets comprising a polyimide resin. On account of a high glass transition temperature (Tg) and a high modulus of elasticity, the polyimide resins are insufficient to mitigate the thermal stress between bonded substrates of semiconductor parts. For reliability enhancement, there is a need for a dicing/die bonding sheet comprising an adhesive composition having a low modulus of elasticity enough for stress mitigation.
On the other hand, film adhesives using heat resistant polyimide resins have been proposed for use in die bonding or lead frame bonding. Most prior art polyimide base adhesives are thermoplastic and thus problematic in that they lose a bonding force at high temperatures as encountered when advanced lead-free solders are used. There is a desire to have a die bonding agent that has a low modulus of elasticity and does not lose a bonding force even at high temperatures during soldering or the like.
Japanese Patent No. 3,221,756 corresponding to U.S. Pat. No. 5,677,393 describes a heat resistant adhesive film comprising a polyimide silicone having phenolic hydroxyl radicals and an epoxy resin. Since phenolic hydroxyl radicals are present at sterically packed positions, their reaction with the epoxy resin is hindered, resulting in insufficient cured properties and insufficient bond strength at high temperatures.